Imaging electron energy filters, also known as electron filters, energy filters, or electron energy spectrometers are used in transmission electron microscopes to improve the contrast of the image of the specimen by selecting electrons of a particular energy range. This is most clearly apparent with non-contrasting, very thin specimens having a selected energy loss (.DELTA.E) in the range of from approximately 100 to 200 eV. However, even an image of a specimen produced with purely elastically scattered electrons (energy loss .DELTA.E=0), which is done by filtering out any non-elastically scattered electrons (.DELTA.E&gt;0), is markedly improved in contrast as compared with the unfiltered image.
A further substantial advantage of a transmission electron microscope having an imaging electron filter is that element-specific images (that is, element distribution images) can be made simultaneously over a relatively wide specimen range in that the energy range allowed to pass through the electron filter corresponds to an element-specific interaction of the transmitted electrons with the specimen, or in other words corresponds to a K, L or M absorption in the shell of the atom. Thus, it is possible to obtain qualitative distribution images, and quantitative distribution images as well if the intensity ratios are measured and the background is subtracted, of the elements in thin specimens (.congruent.30 nm) that have very high local resolution (.congruent.0.5 nm) and maximum detection sensitivity (.congruent.2.10.sup.- g); until now, this was unattainable by any other analysis technology. Maximum local resolution and maximum sensitivity of detection of elements are highly important both in biological and medical research and in the study of materials.
In electron diffraction diagrams, as well, an imaging electron filter produces sharper images of the diffraction diagram by filtering out non-elastically scattered electrons. It is also possible to make diffraction diagrams of non-elastically scattered electrons in a particular energy range.
German Pat. No. 20 28 357 discloses an electron microscope which enables filtering of the image of the specimen or of the image of the diffraction diagram. An electron filter of the so-called Castaing type, comprising a magnetic prism and an electrostatic mirror, is used. The electrostatic mirror, however, is highly sensitive to external stray fields; moreover, at high diffraction voltages for the electrons, problems of insulation also arise. For this reason, purely magnetic filters have been preferred lately. These energy filters are classified as so-called alpha and omega filters in accordance with the course of the electron paths.
An alpha filter is known from a publication by J. P. Perez et al in Journal de Physique, colloquium C2, supplement to n 2, Volume 45, C2-171 (1984). It comprises two magnetic sectors (deflection regions) having different field intensities, which are separated by a narrow interspace, one of the sectors being traversed upon entry into the filter and upon exit therefrom.
To be optimally usable in an electron microscope, an imaging electron filter must meet the following two conditions: first, a large number of image points must be transmitted without loss of local resolution; and second, the imaging errors in the exit source plane (also called a selection plane) must be so slight that narrow energy widths (on the part of the transmitted electrons) can be realized. The known alpha filter does not meet this second condition;